What was the reason for an elongated toilet?

Answer 1

Though it does sound a bit vulgar to say it, since you asked...

Larger men have difficulty fitting their genitals into standard round toilet bowls. They tend to contact the inner surface of the bowl, which is both very unpleasant and unsanitary. In order to keep this from happening, they generally have to sit at an uncomfortable angle and too far back on the seat... which can also cause other unpleasant problems.

The proliferation of elongated toilets is very much appreciated.

A second reason for elongated toilets is they are often perceived as "higher end", for when you are remodeling or installing a bathroom in an expensive house.